Capacitors are commonly employed in ICs for a variety of purposes, such as to condition signals, to store electrical charge, to block DC voltage levels, and to stabilize power supplies. In memory ICs, a capacitor is used to hold enough charge to represent a detectable logic state.
Polysilicon is typically used to construct the electrode plates of the capacitor in a substrate of the IC. The diffusion and doping characteristics of polysilicon result in variable capacitance characteristics, in which the capacitance value varies relative to the voltage level applied to the capacitor and the temperature experienced by the capacitor. Despite the variable characteristics of polysilicon capacitors, the capacitance variation is not of primary concern in digital memory ICs. Memory capacitors are required only to accept charge, to hold some or all of the charge for a finite time period and then discharge, all in a reliable manner. Furthermore, since polysilicon is also used to fabricate other components of the IC, such as transistors and conductors, the plates of the capacitors can be formed simultaneously with the other components of the IC.
In analog and mixed signal circuit applications, on the other hand, capacitors are frequently used as impedance elements whose response characteristic must be linear. If the impedance of the capacitor is not fixed and reliably ascertainable, the capacitance response of the capacitor relative to voltage will vary non-linearly, causing unacceptable variations in the performance of the analog or mixed signal circuit.
Application specific integrated circuits (ASICs) sometimes combine analog circuitry with digital circuitry on the same substrate. In such applications, the fabrication of capacitors has become somewhat problematic. Polysilicon is a semiconductor, which is not the best material to use as an electrode to form a capacitor. A space charge layer forms in the doped polysilicon and adversely affects the capacitance vs. voltage response (linearity) and the frequency response of the capacitor. When a metal material is used for the electrode, however, no space charge layer exists.
Many contemporary ICs employ multiple layers of interconnects, as an adjunct of their miniaturization. Interconnects are layers of separate electrical conductors which are formed overlying the substrate and which electrically connect various functional components of the IC. Because of space and volume considerations in ICs, attention has been focused upon the effective use of the space between the interconnect layers. Normally the space between the interconnect layers is occupied by an insulating material, known as an intermetal dielectric (IMD). One effective use for the space between the interconnect layers is to form capacitors in this space using the interconnect layers. The previously referenced U.S. patent applications focus on different techniques for combining capacitors with the conductors of the interconnect layers to achieve desirable effects within the IC.
Because the conductors of the interconnect layers are of metal construction, the capacitors formed between the interconnect layers are preferably of a metal-insulator-metal (MIM) construction. A MIM capacitor has metal plates, usually formed on the metal conductors of the interconnect layers. The fourth and fifth above identified patent applications describe techniques for forming the metal capacitor plates with the conductors of the interconnect layers. The additional benefit of MIM capacitors is that they possess a higher degree of linearity and an improved frequency response. Unlike polysilicon capacitors, MIM capacitors incorporated within the interconnect levels are unobtrusive to the underlying digital components or circuitry.
The use of a MIM capacitor within the interconnect levels can also reduce the size of the overall IC structure because the digital circuitry exists under the capacitor, instead of beside it. Additionally, MIM capacitors are readily fabricated as part of the interconnect layers without a significant increase in the number of process steps or in the manufacturing costs. Connecting the MIM capacitors in the interconnect layers to the appropriate components of the IC is relatively easily accomplished by post-like or plug-like “via interconnects” that extend between the interconnect layers as needed.
However, even the more linear MIM capacitors are susceptible to non-linear performance under the influence of different electrical and physical conditions, and even relatively small deviations from the expected and desired performance may be sufficient to diminish the effective use of such capacitors in precise linear or analog circuits or in digital circuits.
It is with respect to these and other background considerations that the present invention has evolved.